In a wireless cellular telephony system of the WCDMA kind, only one or a few frequencies are used by the system, which is made possible by the fact that the transmissions in the different cells of the system are made using different so called scrambling codes.
The fact that only one or a few frequencies are used in a WCDMA system may cause a problem when deploying so called Femto Base Stations, which are at present being developed in order to provide wireless cellular telephony coverage for end users in limited coverage areas, so called Femto cells, such as, for example, private homes or offices.
A Femto Base Station will more or less be designed as an “ordinary” base station of the system within which it is intended for deployment, i.e. in this case a WCDMA system. However, a major difference between Femto Base Stations and the “ordinary” or “Macro” base stations of the WCDMA system in which the Femto Base Station is deployed is the output power of the Femto Base Station, and thus the area which can be covered by means of the cell which is served by the Femto Base Station, the Femto cell. The output power of the Femto Base Station will be significantly much smaller than the output power of the Macro base stations, and will thus limit the size of a Femto cell compared to the area of the Macro cells.
The problem referred to above which may arise when using Femto cells interspersed with the larger Macro cells in a WCDMA system is caused by the fact that both the Macro cells and the Femto cells may be deployed on one and the same frequency. Thus, user terminals, UEs, in the Macro cells may interfere with the Femto cells when transmitting to the Macro base stations, i.e. so called Up Link, UL, transmissions.
If a Macro UE is close to a Femto base station, its UL transmissions, which may be made using a high output power level, may cause interference in the Femto base station.
A WCDMA system comprises a control function for Base Stations, known as the Radio Network Controller, the RNC. There is a function in the RNC in a WCDMA system by means of which the RNC keeps track of cells in the vicinity of a cell which it controls, the so called Neighbour list. The RNC also controls certain functions of the UEs in a cell, which is done via the Base Station of the cell.
Potentially, the Neighbour list could be used by an RNC in order to minimize UL interference from UEs to nearby Femto Base Stations. However, it is envisioned that large amounts of Femto cells may be deployed in a WCDMA network. If many Femto cells are deployed, it will not be possible to define all of them as neighbours to surrounding Macro cells, since the number of cells in a Neighbour list may be limited and, in addition, even if that were possible, the work involved in keeping such lists updated would be prohibitive.
Also, the problem described above is underlined by the fact that Femto base stations may be equipped with some kind of access control, since they are intended for use in private home or small offices. This means that it will not be possible for UEs of WCDMA cells in their vicinity to connect to them by means of functions in WCDMA known as soft or softer Hand Over, functions which otherwise might control transmissions of Macro UEs in the vicinity of Femto base stations so that interference problems would not arise.